Studies of the reaction mechanism of the gastric H ion:K ion ATPase will continue using (a) site specific reagents; (b) rapid reaction kinetics; (c) ATP-ADP exchange activity. Gastric vesicle K ion permeability will be studied as a function of putative second messengers on K ion-ATPase activity and will be studied in terms of changes in affinity of the occluding K ion site. The control of the level of K ion-ATPase activity in rat gastric mucosa will be investigated.